Waveguide mode selective absorber



Dec. 8, 1970 QUlNE ETAL I WAVEGUIDE MODE SELECTIVE ABSORBER Filed Aug. 13, 1968 E =S E =Q E =mm t r a I N VENTO S Mai/V3 @l/l/YE C 00618 y Yam 6'5? W77 i? I United States Patent 3,546,635 WAVEGUIDE MODE SELECTIVE ABSORBER John P. Quine, Schenectady, and Coushy Younger, Scotia,

N.Y., assignors to the United States of America as represented by the Secretary of the Air Force Filed Aug. 13, 1968, Ser. No. 752,229 Int. Cl. H01p 1/16 US. Cl. 333-21 4 Claims ABSTRACT OF THE DISCLOSURE The invention comprehends a waveguide component adapted to selectively absorb unwanted TE TM (LSE modes propagating in high power oversized waveguide systems. A section of unslotted hexagonal waveguide is used to convert (LSE modes of electromagnetic wave energy to (LSE modes. The hexagonal waveguide is combined with a section or sections of rectangular waveguide having transverse sidewall slots which etfectively couple out the (LSE Q modes.

BACKGROUND OF THE INVENTION This invention relates to high power oversized waveguide systems and, in particular, to a waveguide mode selective absorber that is capable of selectively absorbing unwanted TE TM modes of electromagnetic wave energy propagating therein.

Mode absorbers are sometimes required in oversized waveguide systems in order to damp out spurious mode resonances. Resonances can occur when a spurious mode becomes trapped within regions of the waveguide system having waveguide cross sectional dimensions which are larger than in the surrounding regions. Trapping can occur, for example, in the region between two tapered transitions.

Mode absorbers are also required in order to prevent the radiation of spurious mode power from antennas connected directly to the output of an oversize waveguide system, that is, without the use of tapers. In this case the mode absorber must provide high enough one way loss to reduce the radiated spurious mode power to acceptable levels; this value of one way loss may be considerably higher than that required to suppress resonance buildup in systems in which spurious mode trapping occurs, and may be obtained only with mode absorbers having inconveniently long lengths. For this reason, the suppression of spurious mode radiation is best accomplished by designing the system components to have the lowest possible spurious mode generation.

When a spurious mode experiences resonances as a result of being trapped, large amounts of power can be converted from the desired mode to the spurious mode. If a mode absorber is not employed, the converted power is dissipated mostly in the waveguide walls within the trapped region; the fields in this case can build up to high values, and overheating and dielectric breakdown can occur.

The principal problem encountered in the design of mode absorbers for ultra-high power waveguide systems is that of producing a required amount of absorption for the spurious mode, with only a negligible loss to the ultra-high power desired mode. The fractional power loss experienced by the desired mode must be very small, if the power dissipated in the mode absorber is to be kept reasonably low. For example, a loss of 0.1 db for the desired mode carrying 100 kw. of average power would require the dissipation of approximately 2.5 kw. in the mode absorber. Obviously, losses which are considerably less than 0.1 db are desirable.

Effective practical absorption of TE TM degenof the short sections of rectangular waveguides that are erate mode pairs has been a long standing problem in the design of high power waveguides. A particular combination of these modes has zero longitudinal current all along the side walls, and therefore cannot always be selectively absorbed by transverse slots on the side walls. To date, investigations have shown that the degeneracy between the TE and TM modes that occurs in rectangular waveguides can be removed by transforming the rectangular waveguide into an hexagonal shape. With the degeneracy removed, no combination of modes results in zero longitudinal side wall currents, and transverse slots on the side Walls can provide mode selective absorption.

These principles have been applied in the current state of the art by the utilization of a hexagonal mode absorber which consists of a length of hexagonal waveguide with slots placed along the centers of the side wall andoriented in the transverse direction. The length of the slots and the slot spacing of such a device are adjusted in order to obtain an optimum conductance per unit length. Because the longitudinal current for the desired TE mode is zero along the center of the side walls of the hexagonal waveguide, the transverse slots do not introduce loss to this mode. With this configuration, the TE TM composite modes having odd n can be absorbed; however, the modes having even 11 cannot be absorbed, because the longitudinal current along the center of the side walls is zero for these modes. The utility of this type of mode absorber is still further diminished because it also requires an impractically long length of hexagonal waveguide.

SUMMARY OF THE INVENTION The state of the art hexagonal mode selective absorber referred to above consists of a length of hexagonal waveguide with transverse slots centered on the side walls to provide coupling to a dissipative medium. These slots provide direct dissipation for only the horizontally polarized (LSE modes, having odd values for the index, n. The vertically polarized (LSE modes are dissipated by first being coupled (because of the hexagonality) to the horizontally polarized (LSE J modes and then being dissipated by coupling through the slots. The subscripts V and H employed above refer to horizontally and vertically polarized modes. The (LSE and (LSE Q modes are combinations of the usual TE and TM modes which have either E or E =0, where E and E are the transverse electric fields. Stated another way, the top wall and sidewall composite modes are defined as the particular combinations of TE and TM modes which have zero net longitudinal current along the top and bottom walls and along the sidewalls, respectively; the top wall and sidewall composite modes can also be called the horizontally and vertically polarized longitudinal section E-modes, respectively.

The present invention also employs a length of hexagonal waveguide. However, it contains no slots. The hexagonal waveguide in this case is employed only as a polarization twisting medium. Thus, an incident (LSE mode is converted partially to an (LSE mode. The conversion occurs in this case also because of the coupling between the (LSE and (LSE modes provided by the hexagonal waveguide. The energy converted to the (LSE J mode is absorbed by means of transverse slots placed on the side walls of sections of attached rectangular waveguide. These sections of rectangular waveguide are connected 'to the hexagonal waveguide by means of hexagonal to rectangular waveguide transition sections.

Mode selective absorption for the (LSE Q and employed in conjunction with the lossless (unslotted) hexagonal waveguide. The important (LSE Q and (LSE modes can, therefore, be absorbed by the device of the present invention as well as the (LSE and (LSE Q modes.

It is a principal object of this invention to provide a new and improved waveguide mode selective absorber for use in high power oversized waveguide systems.

It is another object of this invention to provide an improved hexagonal type waveguide mode selective absorbet.

It is another object of the invention to provide a waveguide mode selective absorber of the type described that utilizes a substantially shorter section of hexagonal waveguide than prior art devices.

It is another object of the invention to provide a waveguide mode selective absorber of the type described that is capable of mode selective absorption for the (LSE and (LSE modes for even as well as odd values of n.

It is another object of the invention to provide a waveguide mode selective absorber of the type described that is more efficient than currently available devices.

These, together with other objects, advantages and features of the invention, will become more apparent from the following detailed description when taken in conjunction with the illustrative embodiments in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a plan view of one presented preferred embodiment of the invention;

FIG. 2 is a sectional view of FIG. 1 taken at II, II;

FIG. 3 is a sectional view of FIG. 1 taken at III, III;

and

FIG. 4 is a sectional view of FIG. 1 taken at IV, 1V.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electric field along the top walls of an unslotted hexagonal waveguide is due entirely to the top wall composite mode, the electric field of the side wall composite mode being zero along the top wall. Furthermore, the composite top wall mode is a characteristic mode of the rectangular waveguide, and consists of the particular combination of TE and TM characteristic modes for which the condition given by is satisfied. In Equation 1, (E and (E are the x components of the transverse electric fields of the components TE and TM modes comprising the top wall composite mode; the x-direction being parallel to the top wall. Equation 1 shows that the net electric field in the x-direction is zero for the composite top wall mode.

The ratio of the y-components of the electric fields of the components TE and TM modes comprising the composite top wall mode is represented by the equation ET: TM P(il TM y)TE P "1 5mm where p and B are the propagation constants of the TE and TM modes in the hexagonal waveguide. Using Equation 2 in Equation 3, there results The mode absorption, A, in a length of waveguide L is defined as the ratio between IE I at 2:0 and [E at z=L. Thus, the difference between p and ,B causes E to vary with z; in the case the quantity, 1[E represents the power converted from the top wall to the side wall composite mode. The mode absorption for Equation 6 shows that A has a minimum value equal to 1.0(0 db) when A0 is an even multiple of 1r, and that A is a maximum when A0 is equal to an odd multiple of 1r. The maximum value, A is given by a 2 mb A =qfr f01 na.

If the ratio the inverse ratio would, of course, be used. The value of A can be determined as a function of frequency and L from Equation 5 by noting that where A is the free space wavelength and (A M and (AQ are the cutolf wavelengths of the TB and TM modes, respectively, in the hexagonal waveguide.

Referring now to FIGS. 1 through 4, there is disclosed thereby a mode suppressor 1 designed in accordance with the principles of the invention and the foregoing equations. Mode suppressor 1 includes hexagonal waveguide section 2, hexagonal waveguide to rectangular waveguide transition sections 3, and rectangular waveguide sections 4. These sections are joined as shown by means of flanges 6 and fastening means 7 in a conventional manner. Rectangular waveguide sections 4 are provided with a series of sidewall slots 5. The dimensions of the waveguide sections and the size and arrangement of the sidewall slots 5 are, of course, determined by the electromagnetic wave frequency for which the system is designed and by the foregoing equations.

A device utilizing but a single slotted rectangular section 4 would be equally effective. With such an arrangement, the rectangular section 4 could be joined to either end of the hexagonal waveguide section 2.

While the invention has been described in one presently preferred embodiment, it is understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects.

What is claimed is:

1. In combination with a high power waveguide system, a mode selective absorber comprising a section of waveguide of hexagonal cross section, a section of waveguide of rectangular cross section, said waveguide of rectangular cross section having a series of transverse sidewall slots therein, and a hexagonal to rectangular waveguide transition section interconnecting said section of waveguide of hexagonal cross section and said section of waveguide of rectangular cross section.

2. A mode selective absorber as defined in claim 1 wherein said section of waveguide of substantially hexagonal cross section has a length elfective to convert (LSE modes of electromagnetic wave energy propagating therethrough to (LSE J modes of electromagnetic wave energy.

3. A mode selective absorber as defined in claim 2 wherein said transverse sidewall slots are operable to absorb (LSE modes of electromagnetic wave energy.

4. A mode selective absorber as defined in claim 3 including a second section of rectangular waveguide having transverse sidewall slots therein and a second hexagonal to rectangular waveguide transition means, said first recited section of rectangular waveguide, said first recited 6 hexagonal to rectangular waveguide transition means, said section of waveguide of a substantially hexagonal cross section, said second hexagonal to rectangular waveguide means, and said second section of rectangular waveguide being connected in contiguous serial relationship within said waveguide system.

References Cited UNITED STATES PATENTS 2,611,087 9/1952 Alford 33321X 3,205,462 9/1965 Meinke 333--98MUX 3,233,241 2/1966 Alford 33321X PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 

